For seat belt apparatuses to be mounted in vehicles such as cars, various seat belt retractors including a pretensioner have been developed. In an initial stage of an emergency, such as a vehicle collision, when a deceleration higher than the normal deceleration is applied to the vehicle, a pretensioner rotates a spool of a seat belt retractor in a seat-belt retracting direction by using reaction gas generated by a gas generator, thereby winding a seat belt around the spool. This quickly removes slack from the seat belt and applies tension to the seat belt so as to increase the occupant restraint force.
As an example of a pretensioner of the related art, there is known a pretensioner in which a plurality of balls serving as force transmission members are stored in a pipe. These balls are moved along the interior of the pipe by receiving gas pressure of reaction gas generated by a gas generator in the event of an emergency, and press a plurality of pressed portions of a ring gear or a driving wheel. When the ring gear or the driving wheel is rotated by this press, a spool rotates in a seat-belt retracting direction to retract the seat belt (for example, see PTL 1 and PTL 2).
In the pretensioner described in PTL 1, the balls whose pressing forces are substantially lost after pressing the ring gear separate from the ring gear, and are received in a ball receiving chamber provided in a case of the pretensioner below the ring gear and substantially shaped like a rectangular parallelepiped. However, when the ball receiving chamber substantially shaped like a rectangular parallelepiped is thus provided below the ring gear, the size of the case of the pretensioner increases, and the size of the seat belt retractor increases inevitably.
In the pretensioner described in PTL 2, the balls whose pressing forces are substantially lost after pressing the driving wheel move with the rotation of the driving wheel, and are stored in a case of the pretensioner while being received in recesses provided on the outer periphery of the driving wheel. According to the pretensioner of PTL 2, since all of the balls whose pressing forces are lost are received in the recesses provided on the outer periphery of the driving wheel, the ball receiving chamber substantially shaped like a rectangular parallelepiped as described in PTL 1 is unnecessary. This can make the case of the pretensioner compact.
In addition, PTL 2 describes that the ball receiving chamber is formed by an arc-shaped curved pipe extending above the case of the pretensioner and outside the case and that the balls whose pressing forces are lost are stored in the pipe. Thus, most of the balls whose pressing forces are lost separate from the driving wheel, and are stored in the curved pipe extending outside the case.